1. Field of the Invention
The invention relates to facsimile modems, and more particularly, enhanced throughput for facsimile modems operating in half-duplex mode.
2. Description of the Related Art
A modem (modulator-demodulator) is a communications device that employs digital modulation techniques to transmit binary data over analog band-limited communications channels. International Telecommunications Union-Telecommunication Standardization Sector (ITU-T) Recommendation V.34 is the standard for modems operating at data signaling rates up to 33,600 bits/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits.
A V.34 modem can operate in full-duplex (FDX) or half-duplex (HDX) mode. Half-duplex describes a mode of operation where exchange of data alternates between unidirectional transmission of primary channel data from source to recipient modem and simultaneous bidirectional transmission of control channel data between the two modems. The primary channel is the main data channel. Together with an optional auxiliary channel data, the primary channel constitutes the bitstream transmitted by the source modem. On the other hand, the control channel is used to exchange information between transmissions of primary channel user data, for example, for changing data signaling rates of the primary channel or the control channel, or exchanging of precoder coefficients.
V.34 modems support adaptive techniques that enable them to achieve close to the maximum data signaling rate the channel can support on each connection. Although the current V.34 modem standard supports a selectable symbol rate of up to 3429 symbols/s, the modem standard does not allow quick speed changes to be made in case of a wrong decision on symbol rate. The present invention is directed toward overcoming or at least reducing the effects of a wrong decision on symbol rate and other applications.
An executable code extension is made to current V.34 modems operating in half-duplex mode that enhances performance and throughput. The technique is useful for facsimile servers and for facsimile transmission over Internet Protocol (xe2x80x9cFAX-over-IPxe2x80x9d) network applications, and the technique allows a compromise between bandwidth and noise performance to be made that optimizes the throughput of a V.34 modem over a given connection. The modem""s primary channel equalizer can be trained to increase the selectable symbol rate from the current maximum of 3429 symbols per second standard to a higher rate of symbol transmission. The technique includes symbol rate re-negotiation and is useful for fast primary channel retrains and for situations in which symbol rates are to be reduced. The technique is also useful for changing the requested symbol rate with data rate negotiation in situations in which data rate selection for the new symbol rate may not be known.
In general, in one aspect, embodiments of the invention provide a modem that is configured, for facsimile operation, to change symbol rate. The modem includes a data pump and a memory device. The memory device is adapted to store, for facsimile operation of the modem, symbol rate re-negotiation code executable by the data pump for re-negotiating symbol rate for a connection with a second modem.